2012A&A...548A..77G

We present the first complete ∼55-671µm spectral scan of a low-mass Class 0 protostar (Serpens SMM1) taken with the PACS and SPIRE spectrometers onboard Herschel. More than 145 lines have been detected, most of them rotationally excited lines of ¹²CO (full ladder from J_u=4-3 to 42-41 and E_u/k=4971K), H₂O (up to 8₁₈-7₀₇ and E_u/k=1036K), OH (up to ²Π_1/2 J=7/2-5/2 and E_u/k=618K), ¹³CO (up to J_u=16-15), HCN and HCO⁺ (up to J_u=12-11). Bright [OI]63, 145µm and weaker [CII]158 and [CI]370, 609µm lines are also detected, but excited lines from chemically related species (NH₃, CH⁺, CO⁺, OH⁺ or H₂O⁺) are not. Mid-infrared spectra retrieved from the Spitzer archive are also first discussed here. The ∼10-37µm spectrum has many fewer lines, but shows clear detections of [NeII], [FeII], [SIII] and [SI] fine structure lines, as well as weaker H₂ S(1) and S(2) pure rotational lines. The observed line luminosity is dominated by CO (∼54%), H₂O (∼22%), [OI] (∼12%) and OH (∼9%) emission. A multi-component radiative transfer model allowed us to approximately quantify the contribution of the three different temperature components suggested by the ¹²CO rotational ladder (T_k^hot≃800K, T_k^warm≃375K and T_k^cool≃150K). Gas densities n(H₂)>5x10⁶cm^–3 are needed to reproduce the observed far-IR lines arising from shocks in the inner protostellar envelope (warm and hot components) for which we derive upper limit abundances of x(CO)≲10^–4, x(H₂O)≲0.2x10^–5 and x(OH)≲10^–6 with respect to H₂. The lower energy submm ¹²CO and H₂O lines show more extended emission that we associate with the cool entrained outflow gas. Fast dissociative J-shocks (v_s>60km/s) within an embedded atomic jet, as well as lower velocity small-scale non-dissociative shocks (v_s≲20km/s) are needed to explain both the atomic fine structure lines and the hot CO and H₂O lines respectively. Observations also show the signature of UV radiation (weak [CII] and [CI] lines and high HCO⁺/HCN abundance ratios) and thus, most observed species likely arise in UV-irradiated shocks. Dissociative J-shocks produced by a jet impacting the ambient material are the most probable origin of [OI] and OH emission and of a significant fraction of the warm CO emission. In addition, H₂O photodissociation in UV-irradiated non-dissociative shocks along the outflow cavity walls can also contribute to the [OI] and OH emission.